This invention relates to a fluorescent x-ray analyzer and more particularly to a fluorescent x-ray analyzer of the wavelength dispersion (WD) type.
A WD type fluorescent x-ray analyzer uses a dispersing crystal to disperse fluorescent x-rays generated by a sample irradiated by x-rays and to introduce diffracted x-rays having a specified wavelength into a detector. For the purpose of wavelength scan, the crystal and the detector are rotated while maintaining a specified angular relationship between them. Explained more in detail, the crystal and the detector are rotated so as to satisfy the Bragg condition given by: EQU 2d sin .theta.=n.lambda. (1)
where d is the lattice interval of the crystal, 2.theta. is the angle of diffraction, .lambda. is the wavelength of the incident fluorescent x-rays and n is the order of diffraction. Since the wavelength of the x-rays entering the detector is gradually changed by such a scan, an x-ray spectrum can be generated by using the scanning angle 2.theta. as the horizontal axis and the x-ray intensity as the vertical axis.
As can be understood from (1), however, it happens sometimes that the first-order diffraction beam of a certain element overlaps more or less at the same angular position of a diffracted x-ray beam of a higher order (that is, the second or higher order) corresponding to another element. In such a situation, since x-ray photons of these mutually overlapping beams have different energies, the detector may use a pulse height analyzer to differentiate between the wave heights of pulse signals by these x-ray photons such that only the pulse signals corresponding to the first-order diffraction beam are counted, thereby eliminating the effects of the higher-order beams and obtaining the x-ray intensity due only to the first-order beam.
Fluorescent x-rays emitted from a single element, however, usually include many characteristic rays having different wavelengths referred, for example, as K.alpha. (more strictly speaking, K.alpha.a and K.alpha.2), K.beta.1, K.beta.2, K.beta.3, L.alpha.1, L2.alpha., etc., corresponding to the electron transitions related to the generation of fluorescent x-rays. Thus, when a sample containing a plurality of different elements is qualitatively or quantitatively analyzed on the basis of its x-ray spectrum, the analysis is carried out by determining which characteristic x-rays of which element are forming each of the peaks in the x-ray spectrum and obtaining the x-ray intensity from the top of the peak.
Although a range in pulse height for analysis is properly selected by means of a pulse height analyzer, however, a portion of the pulse signals due to higher-order x-rays of elements with high contents in the sample may fall within the range set by the pulse height analyzer for selecting the height of pulse signals due to the first-order beams. If the peaks are identified or the peak intensity of fluorescent x-rays is calculated by using an x-ray spectrum (hereinafter referred to as the "first-order beam profile") produced on the basis of pulse signals selected by such a pulse height analyzer, peaks of higher-order beams of other elements may be near the scan angle of the first-order beam of an element of interest. In such a situation, it is not possible to determine from the obtained peak profile alone which element is represented by a given spectrum.
In view of the above, it has been known to obtain a first-order beam profile over a wide range covering almost all elements, to start the identification process from the peaks of elements with short wavelengths such that higher-order beam lines of the other elements are not likely to overlap, and to continue the process sequentially with peaks corresponding to elements with longer wavelengths on the basis of the data on the contained elements which have been identified, checking whether there is any overlapping between the first-order beam lines and higher-order beam lines. For analyzing an overlapping region between a peak corresponding to a first-order beam and peaks corresponding to higher-order beams, it has been known to preliminarily obtain the intensity ratio between them for a target element to be analyzed by making measurements on a standard sample and to carry out the analysis by referring to such ratio.
For preparing a first-order beam profile, use must be made of a crystal with lattice interval such that the condition 2d sin .theta.=.lambda..ltoreq.2d is satisfied because n=1 in Formula (1) above. When a measurement is carried out over a large range of elements including both light and heavy elements, therefore, it is impossible to entirely cover such a wide range of wavelengths by using only one kind of crystal. Thus, it has been necessary to prepare a plurality of crystals having different lattice intervals corresponding to different spectral wavelength ranges and to keep replacing one by another of them as measurements are taken by scanning within specified angular ranges between the detector and the crystal. In other words, the apparatus had to be provided with a plurality of crystals and also with a device for exchanging these crystals. As a result, the apparatus could not be made small and its cost could not be reduced. Moreover, since the scanning must be repeated many times within a same range of angles, a long time was required for the measurement.
A further problem with the prior art technology has been that different experimental data are necessary for different elements for the analysis of the peaks. Even for the analysis of one element, different data are necessary, depending on the condition of the analysis such as the kind of the crystal and the slit. Even if measurement are taken under the same conditions, it is necessary to preliminarily prepare a huge amount of data in order to obtain an accurate result. This means that preparations become an extremely burdensome part of an analysis.